grape pigment (malvidin-3-fructoside) as natural sensitizer …...grape pigment...
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ORIGINAL PAPER
Grape pigment (malvidin-3-fructoside) as natural sensitizerfor dye-sensitized solar cells
N. Gokilamani • N. Muthukumarasamy •
M. Thambidurai • A. Ranjitha •
Dhayalan Velauthapillai
Received: 18 January 2014 / Accepted: 24 May 2014 / Published online: 6 July 2014
� The Author(s) 2014. This article is published with open access at Springerlink.com
Abstract TiO2 nanocrystalline thin films have various
applications, among them dye-sensitized solar cells are
more promising and low-cost alternative to conventional
inorganic photovoltaic devices. TiO2 nanocrystalline thin
films have been sensitized with the natural dye (malvidin-
3-fructoside) extracted from grape fruits. The interaction
between the semiconductor and the dye has been studied.
The maximum absorption band of grape extract at 525 nm
has been shifted to 545 nm after incorporation of the dye
indicating interaction of the dye molecules with TiO2. The
grape extract has polyphenolic anthocyanin pigment
(malvidin-3-fructoside), which has carboxylic and hydro-
xyl groups that can attach effectively to the surface of TiO2
film. The dye-sensitized TiO2-based solar cell sensitized
using grape dye exhibited a Jsc of 4.06 mA/cm2, Voc of
0.43 V, FF of 0.33 and efficiency of 0.55 %. Natural dye-
sensitized TiO2 photo electrodes present the prospect to be
used as an environment-friendly, low-cost alternative
system.
Keywords Nanocrystalline � Sol–gel � TiO2 thin films �Dye-sensitized solar cell
Introduction
Nanocrystalline TiO2 films have attracted scientific and
technological interests because of their potential applica-
tions in the fields of photo electronic optical devices, solar
cells, gas sensors, photocatalysts and biomaterials [1–3].
TiO2 is the best photo-catalyst so far, that absorbs the light
at shorter wavelength below 400 nm, due to its wide band
gap. Dye-sensitized solar cell, a device converting light
energy to electrical energy by imitating photosynthesis of
plants [4], was firstly developed by Gratzel’s group [5–7].
It is widely known as a low-cost and easy-assembly solar
cell, in which both synthetic and natural dyes can be used
as a sensitizer. Electrical energy is generated when the cell
is exposed to sunlight. Electrons in dye molecules are
excited and then injected to the conduction band of a wide
band gap (TiO2) n-type semiconductor on which the dye
molecules adsorb. These electrons migrate through the host
semiconductor particles until they reach the collector; also
the holes simultaneously generated are reduced by a redox
electrolyte or hole carrier at the back electrode (Pt).
Because of low material cost, simple preparation, and lack
of heavy metals, natural dyes are more favorable to be
applied in such device [8–12].
Solar cells are the basic block of solar array where the
absorption of light quanta of specific energy results in
generation of charge carriers. Due to absorption of sun
light, dye molecules get excited from the highest occupied
molecular orbital’s (HOMO) to the lowest unoccupied
molecular orbital’s (LUMO).
D þ hv ! D� ð1Þ
Once an electron injected into the conduction band of
the wide band gap semiconductor nano structured TiO2
film, the dye molecule (photosensitizer) become oxidized.
N. Gokilamani (&) � N. Muthukumarasamy � A. Ranjitha
Department of Physics, Coimbatore Institute of Technology,
Coimbatore, India
e-mail: [email protected]
M. Thambidurai
Department of Electrical and Computer Engineering, Global
Frontier Center for Multiscale Energy Systems, Seoul National
University, Seoul 151-744, Republic of Korea
D. Velauthapillai
Department of Engineering, University College of Bergen,
Bergen, Norway
123
Mater Renew Sustain Energy (2014) 3:33
DOI 10.1007/s40243-014-0033-6
D� þ TiO2 ! Dþ þ e� TiO2ð Þ ð2Þ
The injected electron is transported between the TiO2
nanoparticles and then gets extracted to a load where the
work done is delivered as an electric energy.
e� TiO2ð Þ þ Electrode ! TiO2 þ e� Electrodeð Þ þ Energy
ð3Þ
To mediate electron between the TiO2 photoelectrode
and the platinum counter electrode, electrolyte containing
I-/I�3 redox ions is used to fill the cell. Therefore, the
oxidized dye molecules (photosensitizer) are regenerated
by receiving electrons from the I- ion redox mediator that
will oxidized to I�3 (Tri-iodide ions).
Dþ þ e� ! D þ I�3 ð4Þ
The I�3 substitutes the donated electron internally with
that from the external load and gets reduced back to I- ion.
2e� Electrodeð Þ þ I�3 ! 3I� þ Electrode ð5Þ
Therefore, generation of electric power in dye-sensitized
solar cell causes no permanent chemical change or
transformation. The conducting electrodes are prepared
such that they posses low-sheet resistance and very high
transparency in order to facilitate high solar cell
performance [13]. Figure 1 shows the structure of a dye-
sensitized solar cell.
Anthocyanins consists a large family of widespread
flavonoids in plants and they are responsible for many fruit
and floral colors that are observed in nature. Chemically,
these flavonoids are most commonly based on six antho-
cyanidins: pelargonidin, cyanidin, peonidin, delphinidin,
petunidin and malvidin [14]. Interest in the anthocyanins
has increased because of their potential as natural colorants
and non-toxicity. They have bright attractive colors, and
they are water-soluble. Acylated anthocyanin pigments
show greater stability during processing and storage.
Because of the presence of acylated groups in the structure
of anthocyanins, it is believed that they can protect the
oxonium ion from hydration, thereby prevent the formation
of hemiketal (pseudobase) or chalcone forms. Anthocyanin
pigments were found in 1835 for the first time. In 1916,
Wehldale proposed anthocyanin flavanons formation
pathway, since they were products of shikimic acid cycle.
Willstatter and Zollinger [15, 16] reported malvidin
3-fructoside as the major pigment in grapes, and malvidin
3,5-diglucoside and malvidin as the minor ones. Antho-
cyanins from grapes include mono and diglucosides of five
different aglycones with the addition of monoacylation
[14]. The structure of malvidin 3-fructoside is shown in
Fig. 2.
Sol–gel dip coating method has been used because of the
low-cost equipments involved, lower growth temperatures
and reproducibility. Many sensitizers have been used by
different workers to sensitize TiO2 nanoparticles for solar
cell applications. In this paper, we report a simple route to
synthesize TiO2 nanoparticles. TiO2 nanoparticles have
been used as photoelectrode material to fabricate the dye-
sensitized solar cells. The use of non-toxic natural pig-
ments as sensitizers would definitely enhance the envi-
ronmental and economic benefits of this alternative form of
solar energy conversion. Natural dye-sensitized solar cells
are a promising class of photovoltaic cells with the capa-
bility of generating green energy at low production cost
since no vacuum systems or expensive equipment are
required in their fabrication. In addition, natural dyes are
abundant, easily extracted and safe materials.
Experimental
To prepare the photo-anode of dye-sensitized solar cells,
the indium-doped tin oxide conducting (ITO) glass sheet
Fig. 1 Schematic diagram of dye-sensitized solar cell
Fig. 2 Structure of malvidin-3-fructoside
Page 2 of 7 Mater Renew Sustain Energy (2014) 3:33
123
(Asahi Glass; Indium-doped SnO2, sheet resistance: 15 U/
square) was first cleaned in a detergent solution using an
ultrasonic bath for 15 min, rinsed with double distilled
water and then dried. The matrix sol was prepared by
mixing 0.695 ml of titanium (IV) isopropoxide with 15 ml
of isoproponal at room temperature and stirred for half an
hour. 0.135 ml of glacial acetic acid is added drop wise and
stirred vigorously for 2 h to obtain a homogeneous mixture
of TiO2 sol. The prepared sol was deposited on the Indium-
doped tin oxide (ITO) glass plate by sol–gel dip coating
method. The film was dried at 80 �C for 30 min in air and
then annealed at 500 �C in a muffle furnace. For grapes a
dye extract preparation, well cleaned grape fruits were
mixed with 250 ml ethanol and were kept for 12 h at room
temperature. Then residual parts were removed by filtra-
tion. The ethanol fraction was separated and few drops of
concentrated HCl was added so that the solution became
deep red color (pH \ 1). This was directly used as dye
solution for sensitizing TiO2 electrodes.
Crystallinity and phase analysis of the films were carried
out using X-ray diffraction method (Rigaku Rint 2000
series). Energy dispersive X-ray analysis (EDAX) and
structural studies have been carried out using transmission
electron microscope (JEOL, JEM-2100). Transmission
spectra have been recorded using UV–VIS–NIR spectro-
photometer (Jasco V-570). Lithium iodide, iodine and
acetonitrile purchased from Sigma-Aldrich have been used
as received for the preparation of electrolyte. The redox
electrolyte with [I�3 ]/[I-] 1:9 was prepared by dissolving
0.5 M LiI and 0.05 M I2 in acetonitrile solvent. Since LiI is
extremely hygroscopic, electrolytes were prepared in a dry
room maintained at dew point of 60 �C. The counter
electrode was prepared by using platinum chloride as fol-
lows: the H2PtCl6 solution in isopropanol (2 mg/ml) was
deposited onto the ITO glass by spin coating method. TiO2
electrode annealed at 500 �C was immersed in the
extracted dye solution at room temperature for 24 h in the
dark, since the grape dye extract is not stable under pro-
longed illumination of light. The electrode was then rinsed
with ethanol to remove the excess dye present in the
electrode and then the electrode was dried. The counter
electrode was placed on the top of the TiO2 electrode, such
that the conductive side of the counter electrode facing the
TiO2 film with a spacer separating the two electrodes. The
two electrodes were clamped firmly together using a binder
clip. Now the prepared liquid electrolyte solution was
injected into the space between the clamped electrodes.
The electrolyte enters into the cell by capillary action. This
resulted in the formation of sandwich type cell. Natural
dye-sensitized TiO2-based solar cells have been fabricated
with area of 0.25 cm2, and it was found that the cell effi-
ciency was independent of cell area in this range as
reported by Yamazaki et al. [17]. The J–V characteristics
of the cell were recorded using a Keithley 4200-SCS meter.
A xenon lamp source (Oriel, USA) with an irradiance of
100 mW/cm2 was used to illuminate the solar cell
(equivalent to AM1.5 irradiation).
Results and discussion
Figure 3 shows the X-ray diffraction pattern of the sol–gel
prepared TiO2 films, annealed at 500 �C. A narrow peak at
25.35� corresponding to (101) reflection of the anatase
phase of TiO2 has been observed in the diffraction pattern.
The observed peaks corresponds to (1 0 1), (0 0 4), (2 0 0),
(1 0 5), (2 1 1) and (2 0 4) which represent only anatase
phase of TiO2. No peaks corresponding to the rutile or
brookite phase has been observed. The grain size has been
calculated using Scherer’s formula [18].
D ¼ Kk=bcosh ð6Þ
where D is the grain size, K is a constant taken to be 0.94, kis the wavelength of the X-ray radiation (k = 1.51 A
´), b is
the full width at half maximum and h is the angle of dif-
fraction. The grain size was found to be 33 nm for the film
annealed at 500 �C. The annealing temperature facilitates
the subsequent crystal growth process, accompanied by the
diffusion of titania species forming big size anatase crystals
formed by merging of some adjacent mesopores. At the
same time, the spatial confinement by mesopore arrays
controls the formation and growth of anatase phase, leading
to a more or less uniform distribution of titania
nanocrystals.
Energy dispersive X-ray (EDAX) spectra of nanocrys-
talline TiO2 thin film is shown in Fig. 4. The compositional
analysis of TiO2 thin films shows the presence of 32.7 at%
10 20 30 40 50 60 70 80
(204
)
(105
)
(200
)
(004
)
(101
)
Inte
nsi
ty (
a.u
.)
2θ (degrees)
Fig. 3 X-ray diffraction pattern of annealed at 500 �C TiO2 thin
films
Mater Renew Sustain Energy (2014) 3:33 Page 3 of 7
123
of Ti and 67.3 at% of O. Figure 5 shows the transmission
electron microscope images of the TiO2 thin film annealed
at 500� C. Figure 5a shows the presence of close-packed
agglomerated nanoparticles of uniform size which causes
the mesoporous structure. This accumulation of nanopar-
ticles creates narrow channels that may serve as electronic
injection membranes [19]. The mesoporous structure,
which provides a large surface area for adsorbing the dye,
has been achieved in the present study, although no poly-
mer was added to make pores. The prepared TiO2 films
exhibited good mechanical strength, had good adherence to
the substrate and could not be easily erased by hand. Fig-
ure 5b shows the high-resolution transmission electron
microscope (HRTEM) image of TiO2 thin films. The
interplanar distance has been calculated using the lattice
fringes and is found to be 0.33 nm which corresponds to
(101) lattice plane of anatase phase. Selective area electron
diffraction pattern is used to learn about the crystal prop-
erties of a particular region. Figure 5c shows the selective
area electron diffraction pattern of the nanocrystalline TiO2
thin film and the presence of rings with discrete spots
suggest that the TiO2 nanocrystalline film is made of small
particles of uniform size with anatase phase.
Figure 6a–c shows atomic force microscope images of
the TiO2 thin films annealed 400, 450 and 500 �C. The
image shows well-defined particle-like features with
granular topography and indicates the presence of small
crystalline grains. Because of the heat treatment, the nano
crystalline phase has been formed and this has led to the
appearance of grains making the films to have higher sur-
face roughness. The root mean square surface roughness of
the film was found to be 19, 27 and 32 nm for the TiO2
films annealed at 400, 450 and 500 �C.
To discriminate the local order characteristics of the
TiO2 films, we carried out non-resonant Raman
spectroscopy studies. The technique is non-destructive,
capable to elucidate the titania structural complexity as
peaks from each crystalline phase is clearly separated in
frequency, and therefore the anatase and rutile phases are
easily distinguishable. Figure 7 shows the Raman spectra
of the nanocrystalline TiO2 sample annealed at 500 �C.
Raman peaks originate from the vibration of molecular
Fig. 4 The EDAX spectra of TiO2 thin film
Fig. 5 a TEM image b HRTEM image and c SAED pattern of TiO2
nanocrytalline thin film annealed at 500 �C
Page 4 of 7 Mater Renew Sustain Energy (2014) 3:33
123
bonds, that is, vibrational mode Eg and A1g peaks, which
are related to different crystal planes. According to factor
group analysis, anatase has six Raman active modes
(A1g ? 2B1g ? 3Eg). Ohsaka et al. [20] have reported the
Raman spectra of an anatase TiO2 and have stated that six
allowed modes appear at 144 cm-1 (Eg), 197 cm-1 (Eg),
399 cm-1 (B1g), 513 cm-1 (A1g), 519 cm-1 (B1g), and
639 cm-1 (Eg). The vibrational peaks of the nanocrystal-
line TiO2 samples at 143, 197, 396, 519, 638 cm-1 and the
absence of overlapped broad peaks show that the material
is well crystallized, with low number of imperfect sites.
These peaks are unambiguously attributed to the anatase
modification. A special attention must be given to the
Raman peaks observed at 143, 197, 396, 519, 638 cm-1
which are all slightly shifted, probably due to a smaller size
of TiO2 nanocrystalline particles. The peak at 513 and
517 cm-1 are very close to each other. The main spectral
features of samples annealed at different temperatures are
closely similar which mean that the prepared samples
posses a certain degree of long range order of the anatase
phase. The spectra vary symmetrically with grain size. It
deteriorates with spectroscopic lines broadening and
merging, line intensity decreases and position shifting with
decrease in annealing temperature. However, the spectrum
of sample showing the band broadening with decrease in
intensity [21].
Optical absorption spectra of grape fruits and grape
fruits absorbed TiO2 nanocrystalline thin films are shown
in Fig. 8. It is seen that the absorption peak of grape fruits
extract is maximum at 510 nm. The presence of the
absorbance peak at 530 nm in grape fruits sensitized TiO2
nanocrystalline thin films confirms the incorporation of dye
into the TiO2 film. Malvidin-3-fructoside flavanoid is
responsible for the absorption of light. This chemical
attachment affects the energy levels of the highest occupied
molecular level (HOMO) and the lowest unoccupied
Fig. 6 AFM images of TiO2 thin films annealed at a 400 �C, b 450 �C and c 500 �C
0 200 400 600 800 1000
(Eg)
Inte
nsi
ty (
a.u
.)
Raman Shift (cm-1)
(Eg)
(B1g) (Eg)
(A1g +B1g)
Fig. 7 The Raman spectra of TiO2 thin film annealed at 500 �C450 475 500 525 550 575 600 625 650
0.2
0.4
0.6
0.8
1.0
Ab
sorb
ance
(a.
u)
Wavelength (nm)
grapes dyegrape +TiO2
Fig. 8 Absorption spectra of grapes sensitized TiO2 thin films
Mater Renew Sustain Energy (2014) 3:33 Page 5 of 7
123
molecular level (LUMO) of the anthocyanidin molecule
[22], which eventually affects the band gap of these
materials and this results in a shift in the absorption peak of
the absorption spectra. It is seen that absorption coefficients
of grape fruits extract is about 14 times higher than that of
the N-719 dye used in high efficiency dye-sensitized solar
cells [23]. The absorption bands of the dye adsorbed TiO2
semiconductor films are shifted to longer wavelength when
compared to the absorption spectra of the dye solution as
shown in Fig. 8. The intensity of light absorption has been
enhanced due to the interfacial Ti–O coupling which exists
between the C=O, C–H, C–O, O–H bonding of dye mol-
ecules and TiO2 molecules [24].
Figure 9a, b shows the Fourier transform infrared
spectra (FTIR) of grape extract, grape extract-sensitized
TiO2 nanocrystalline thin films. The spectra have been
recorded using FTIR 1600 infrared spectrophotometer,
operating in the wave number range 800–4,400 cm-1. For
the grape extract, the bands in the region of 1,651 cm-1
represents the amide group and the bands in the regions
2,900, 2,978 cm-1 represents the C–H bonding of the
grape extract and a broad sharp peak is obtained in the
region 3,371 cm-1 which corresponds to the hydrogen
bonding. The bands in the region 1,381 cm-1 correspond
to OH bonding. The bands in the region 1,273 and
1,087 cm-1 correspond to O–C bonding and the band at
879 cm-1 represents the C–H bonding in the grape extract.
The grape extract-sensitized TiO2 nanocrystalline thin
films shows a peak at 1,442 cm-1 which corresponds to the
absorption band of TiO2 molecules [25]. The bands ranging
from 2,854 to 3,363 cm-1 correspond to the C–H stretch-
ing due to methyl and methylene groups.
The J–V characteristics of TiO2 nanocrystalline thin
films sensitized with natural dyes is shown in Fig. 10. The
overall efficiency g of the grape dye cells are evaluated in
terms of open circuit voltage (Voc), short circuit current
(Jsc) and fill factor (FF) using the formula
FF ¼ Jmax � Vmax
Jsc � Voc
ð7Þ
g ¼ Jsc � Voc � FF
Pin
ð8Þ
where Jsc is the short circuit photocurrent density
(mA cm-2), Voc the open circuit voltage (volts), Pin is the
intensity of the incident light (Pin = 1 Wcm-2) and Jmax
(mA cm-2) and Vmax (volts) are the maximum current
density and voltage in the J–V curve, respectively, at point
of maximum power output. The solar cell sensitized with
grape extract dye exhibited a power conversion efficiency
of 0.55 % with a short circuit current density (Jsc) of
4.06 mA/cm2, open circuit voltage (Voc) of 0.43 V and fill
factor (FF) of 0.33.
Conclusion
TiO2 nanocrystalline thin films have been prepared by a
simple sol–gel method. X-ray diffraction analysis reveals
that the TiO2 nanocrystalline thin films exhibit anatase
structure. The dyes extracted from grapes strongly absorb
visible light at 510 nm and the absorption bands of the dye
adsorbed TiO2 semiconductor films are shifted to longer
wavelength and have been found to be suitable for the use
as sensitizer in solar cells. The efficiency of the fabricated
dye-sensitized solar cell using grapes extract is 0.55 %.
Unlike artificial dyes, the natural ones are available, easy to
prepare, low in cost, non-toxic, environmentally friendly
and fully biodegradable.
1000 1500 2000 2500 3000 3500 400086
88
90
92
94
96
98
100
102
3363
(b)
Tra
nsm
itta
nce
(%
)
Wavenumber (cm )-1
(a)
3371
2978
2900
1730
1622
1442
1381
1273
1087
1057
879
1087
2306
1651
2854
Fig. 9 FTIR spectra of a grape extract, b grape extract-sensitized
TiO2
-0.2 -0.1 0.0 0.1 0.2 0.3 0.4 0.5
-0.005
-0.004
-0.003
-0.002
-0.001
0.000
0.001
Cu
rren
t d
ensi
ty (
A/c
m2)
Voltage(V)
Voc =0.43 V Jsc =4.06 mA/cm2
FF = 0.33 = 0.50%η
Fig. 10 J–V characteristics of dye-sensitized TiO2-based solar cells
Page 6 of 7 Mater Renew Sustain Energy (2014) 3:33
123
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